Control method of searching neighboring cells, mobile station, and mobile communication system

ABSTRACT

A control method of searching for a neighboring cell of a mobile station communicating with a base station is provided in a direct sequence CDMA mobile communication system which transmits information by carrying out double modulation using a first spreading code group and a second spreading code. The first spreading code group includes spreading codes that have a same repetition period as an information symbol period and are used in common by the base stations, and the second spreading code has a repetition period longer than the information symbol period. The base stations are assigned different second spreading codes. The control method stores at least one second spreading code and its phase into a first table, which second spreading code corresponds to a perch channel whose second spreading code and phase are known; stores a second spreading code used by a neighboring base station into a second table; searches for a perch channel whose second spreading code and phase are unknown; and searches for a perch channel whose second spreading code and phase are known. The neighboring cell search method can save the power consumption and time required for the mobile station to carry out the cell search with preventing an increase in the total cost of the system.

This application is based on Japanese Patent Application Nos. 11-252294(1999) filed Sep. 6, 1999 and 11-260409 filed Sep. 14, 1999, thecontents of which are incorporated hereinto by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a mobile communication system forcarrying out multiple access using spread spectra, and particularly to aneighboring cell search method applied to handover control duringcommunication or to zone re-selection control during the idle mode inthe system and a mobile station constituting the system.

2. Description of the Related Art

A mobile communication system like a widespread mobile phone systemoffers its services by dividing the entire service area into rathersmall radio zones called cells. As shown in FIG. 1, such a systemcomprises a plurality of base stations 111 for covering divided radiozones (cells), and mobile stations 112 for communicating with the basestation by estabishing radio channels. Direct Sequence CDMA (DS-CDMA) isa scheme for a plurality of users to carry out communications using thesame radio frequency band by transmitting information through secondmodulation that spreads a conventional information data modulationsignal with a high rate spreading code. The radio signal of each user isidentified by a spreading code assigned to the user.

In the mobile communication system, the spreading code used for thespreading usually consists of a combination of two types of spreadingcodes: a “first spreading code group” with the same period as aninformation symbol period and commonly assigned to all the basestations; and a “second spreading code” with a considerably longerperiod than the information symbol period and uniquely assigned to eachof the base stations.

FIG. 2 is a schematic diagram illustrating a method of using thespreading codes in the mobile communication system to which the presentinvention is applied. In FIG. 2, the upper layer represents a scramblingcode layer 202 with a long period uniquely assigned to individual basestations, and the lower layer represents a channelization code layer 204with a short period commonly assigned to all the base stations. Thesignals transmitted from the base stations are identified using longperiod scrambling codes uniquely assigned to the individual basestations. A plurality of codes are defined as the scrambling codes forthe entire system, and system designers select the codes to be assignedto the base stations among them.

For the mobile stations to demodulate information transmitted from thebase stations, they must receive the information in synchronization withthe timing of the spreading code repeated periodically at thetransmitting side. In particular, as for the scrambling codes, detectionof the timing requires a long time because of the long period.Accordingly, it is important for the mobile station to detect therepetition timing of the scrambling codes to demodulate perch channelsof the base stations. In the present specification, the repetitiontiming of the scrambling codes is referred to as “phase”. It is notnecessary to detect the absolute phase in practice, but to find therelative difference in the timing between the scrambling codes of thebase stations, that is, the phase difference. Thus, the term “phase”refers to the relative phase between the scrambling codes in the presentspecification.

FIG. 3 is a schematic diagram illustrating timing relationships betweenthe scrambling codes associated with signals sent from the base stationsto a mobile station.

FIG. 3 illustrates a case of an inter-cell asynchronous mobilecommunication system, in which synchronization between the base stationsare not necessarily required, and the timing of the scrambling codesreceived by the mobile station differs for each base station. On thecontrary, in an inter-cell synchronous system establishingsynchronization between the base stations, the timing of the scramblingcodes is exactly adjusted to the timing assigned in advance to the basestations. Accordingly, the relative timing of the scrambling codesbetween the base stations is fixed and unchangeable. Comparing theinter-cell asynchronous system with the inter-cell synchronous system,the former has an advantage over the latter that it does not require anytiming source such as the GPS (Global Positioning System) which isnecessary for the synchronous system, and hence is more flexible inextending the system or the like.

The radio signal transmitted from a base station at certain transmissionpower travels through space with a certain attenuation, and arrives at areceiving site. Since the attenuation the radio signal undergoesincreases with the distance from the transmitting site to the receivingsite, it is common that a perch channel transmitted from a distant basestation is received at a lower received level, and a perch channeltransmitted from a near base station is received at a higher receivedlevel. In practice, however, the propagation loss is not determined onlyby the distant, but varies because of such conditions as the geographyand buildings. As a result, the received power of the perch channelsfrom the base stations fluctuate sharply with the move of the mobilestation. In the condition in which the received levels of the perchchannels from the base stations fluctuate sharply, perch channelsreceived above a certain required received level alter incessantly. Thisis because the received level of the current perch drops suddenly, orthe received level of a perch unreceivable increases abruptly above thereceivable level. Thus, to receive the signals from the base stationswith better quality, it is important for the mobile station tocontinuously monitor the perches from the base stations, and to selectthe best base station.

In the asynchronous mobile communication system, a mobile station mustsearch for a perch quickly whose spreading code and phase are unknown.As a method of searching for a phase, there is one called “3-step cellsearch” disclosed in a document by K. Higuchi, M. Sawahashi go, and F.Adachi, “Fast Cell Search Algorithm In Inter-Cell Asynchronous DS-CDMAMobile Radio”, IEICE Trans. Commun., Vol. E81-B, No.7, July 1998. Themethod provides a “masked symbol” to part of the perch channel whichundergoes double spreading by a channelization code and a scramblingcode. Here, the “masked symbol” is spread only by the channelizationcode without using the scrambling code.

FIG. 4 is a schematic diagram illustrating a structure of a perchchannel.

First, the mobile station despreads the received signal using achannelization code 404 commonly used by all the base stations. Thisenables the mobile station to detect a peak at the timing of a maskedsymbol 408 of the received signal independently of the types of thescrambling codes (first step).

Subsequently, in response to the timing extracted at the first step, themobile station detects a scrambling code group code 406 superimposed atthe same position as the masked symbol 408, and identifies the group towhich the scrambling code belongs which is used by the base station inconnection with the reception (second step).

Finally, using the scrambling codes belonging to the group determined atthe second step, the mobile station identifies the scrambling code 402used by the base station (third step).

In the system to which this method is applied, a lot of scrambling codesare divided into groups in advance. In contrast, in the inter-cellsynchronous system, since the phase differences of the scrambling codesbetween the base stations are known in advance, and hence the searchingtiming can be limited to a fixed timing width (search window), the powerconsumption or time taken for the cell search can be saved.

The conventional search method in the inter-cell asynchronous system,however, requires more power consumption and time for the cell searchthan the inter-cell synchronous system, presenting a problem ofexhausting the battery power of the mobile terminal quickly. On theother hand, employing the inter-cell synchronous system to simplify thecell search of the mobile station presets problems of hindering makingfull use of the above mentioned advantages of the inter-cellasynchronous system, and of increasing the cost of the total system.

As described above, the mobile communication system such as a currentlywide spread mobile phone system comprises the plurality of base stations111 for covering divided radio zones, and the mobile stations 112 forcommunicating with the base stations by establishing radio channels asshown in FIG. 1.

The radio signal transmitted from a base station at certain transmissionpower travels through space with a certain attenuation, and arrives at areceiving site. Since the attenuation the radio signal undergoesincreases with the distance from the transmitting site to the receivingsite, a perch channel transmitted from a distant base station is usuallyreceived at a lower received level, and a perch channel transmitted froma near base station at a higher received level. In practice, however,the propagation loss is not determined only by the distant, but variesbecause of such conditions as the geography and buildings. As a result,the received power of the perch channels from the base stationsfluctuates sharply with the move of the mobile station. Thus, to receivethe signals from the base stations at better quality, it is importantfor the mobile station to continuously monitor the perch channels fromthe base stations, and to select the best base station. To select thebest base station, the mobile station must continuously confirm thepropagation condition of a captured perch channel, or search foruncaptured new perch. Such confirmation of the propagation state of thecaptured perch channel and the search for the uncaptured new perchchannel are generically called “quality measurement of the perchchannel” in the present specification.

On the other hand, a technique called intermittent reception is appliedto the mobile station to prolong the life of the battery by reducing thepower consumption. Although the mobile station in an idle mode mustcontinuously monitor the paging, the intermittent reception halts thereceiver as much as possible when unnecessary to receive, thereby savingthe power consumption.

FIG. 5 is a schematic diagram illustrating a structure of a pagingchannel defined by ARIB IMT-2000 Study Committee, “Japan's RevisedProposal for Candidate Radio Transmission Technology on IMT-2000: W-CDMARevised Proposal Version 1.1” (September, 1998, ARIB). According to thispaper, to increase the effect of the intermittent reception, the pagingchannel is structured such that multiple mobile stations are dividedinto a plurality of groups, and paging signals for respective groups aremapped onto a single physical channel. FIG. 5 illustrates a pagingsignal assigned to one of the groups. In FIG. 5, reference symbols PIseach designate a very short signal informing whether paging is presentor not. Reference symbols MUIs each designate a portion including paginginformation (ID number of mobile station). In FIG. 5, such aconfiguration is assumed in which the PIs are transmitted twice (PI1 andPI2) to improve the receiving accuracy of the PI, and four pieces ofpaging information (MUI1-MUI4) can be transmitted for four mobilestations per group. In other words, the paging signal consists of twoPIs and four MUIs, and receiving time period per paging signal is about15 milliseconds. The paging channel consists of multiplexed pagingsignals with the same structure, the number of which equals the numberof the groups. FIG. 5 illustrates that the mobile station receives thepaging signal of its own group at every 720 millisecond interval.

The mobile station receives the PI portion, first, and then the MUIportion only when a decision is made that the paging is present as aresult of receiving the PI portion. This offers two advantages: First,it is enough for the mobile station to receive only the paging of itsown group; and second, to receive only the PI portion when there is notpaging information. This in turn can limit the actually requiredreceiving time rate to a small value, making it possible to reduce thepower consumption to a very small amount.

FIG. 5 illustrates the paging information which is already transmittedfrom the base station and selected through the decision by the mobilestation. In an actual situation, however, the mobile station must searchfor the perch channels of the neighboring base stations as it moves.Since the mobile station must receive a lot of receivable perch channelsto search for the neighboring base stations, it is important to minimizethe frequency of the search operation to increase the effect of theintermittent reception.

Thus, to select the best base station with the movement of the mobilestation, there is a trade off between the continuous monitoring of theperch channels of the neighboring base stations by searching andreceiving them, and the reduction in the operation time rate of thereceiver to prolong the battery of the mobile station as long aspossible. On the one hand, the reduction in the operation time rate ofthe receiver will results in a decrease in the selection accuracy of thebase station, bringing about undesirable results such as degradation inthe service quality. On the other hand, an increase in the operationtime rate of the receiver to improve the selection accuracy of the basestation will consume the battery of the mobile station quickly,presenting a problem of markedly impairing the usefulness of the mobilestation. In the conventional cell search control method, however, thequality measurement of the perch channel is implemented periodically asdescribed in the following paper with considering the trade off betweenthe selection accuracy of the base station and the service quality: K.Yunoki, A. Higashi and N. Tsutsumi, “Cell Search Strategy on W-CDMAMobile Station”, B-5-186 of the 1999 IEICE General Conference.Specifically, since the quality measurement of the perch channel iscarried out independently of the reception of the paging signal, themobile station must operate its receiver at both timings of qualitymeasurement of the perch channel and the reception of the paging signal,which presents a problem of increasing the consumption of the battery,one of the essential resources of the mobile station.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a controlmethod of searching for a neighboring cell, and a mobile station in aninter-cell asynchronous system with taking full advantage of the system,and saving the power consumption and time required for the cell searchby the mobile station without increasing the total cost of the system.

Another object of the present invention is to save the power consumptionwith maintaining the accuracy of selecting the best base station in themobile communication system that comprises a plurality of base stationsand mobile stations communicating with the base stations by means of thecode division multiple access. This is implemented by the mobile stationby receiving perch channels transmitted from the base stations, bydeciding the base station the mobile station should wait for orcommunicate with, and by monitoring the paging signal to the mobilestation itself by the intermittent reception in the idle mode.

In the first aspect of the present invention, there is provided acontrol method of searching for a neighboring cell of a mobile stationcommunicating with base stations in a direct sequence CDMA mobilecommunication system which transmits information by carrying out doublemodulation using a first spreading code group and one of secondspreading codes, the first spreading code group including spreadingcodes that have a same repetition period as an information symbol periodand are used in common by the base stations, the second spreading codeshaving a repetition period longer than the information symbol period,and being different for each of the base stations, the control method ofsearching for a neighboring cell comprising:

a step of storing into a first table the second spreading code and itsphase of at least one perch channel, which second spreading code andphase are known;

a step of storing a second spreading code used by a neighboring basestation into a second table;

a first search step of searching for a perch channel whose secondspreading code and phase are unknown; and

a second search step of searching for a perch channel whose secondspreading code and phase are known, wherein

the control method of searching for a neighboring cell carries out thefirst search step and the second search step using the first table andthe second table.

Here, the control method of searching for a neighboring cell may furthercomprise the step of transferring, when capturing a perch channel in thefirst search step of searching for a perch channel whose secondspreading code and phase are unknown, the second spreading codecorresponding to the perch channel from the second table to the firsttable.

The control method of searching for a neighboring cell may furthercomprise the steps of:

carrying out the second search step using the first table;

carrying out the first search step using the second table; and

detecting a new perch channel by comparing a search result at the secondsearch step with a search result at the first search step.

The first search step may comprise:

a step of detecting a peak of a received signal at a timing of a maskedsymbol of the received signal by despreading the received signal usingthe first spreading code group;

a step of identifying a group to which the second spreading codebelongs; and

a step of identifying the second spreading code. The control method ofsearching for a neighboring cell may comprise the steps of:

carrying out the second search step using the first table;

carrying out a third search step using the second table, the thirdsearch step consisting of part of sub-steps constituting the firstsearch step;

deciding detection of a new perch channel by comparing a search resultof the second search step with a search result of the third search step;and

carrying out a fourth search step in response to a decision result, thefourth search step consisting of sub-steps of the first search step,which are not carried out in the third search step.

The direct sequence CDMA mobile communication system may spreadinformation into a signal with a bandwidth broader than a frequencybandwidth of the information using a spreading code sequence with a ratehigher than an information transmission rate.

In the second aspect of the present invention, there is provided amobile station communicating with base stations in a direct sequenceCDMA mobile communication system which transmits information by carryingout double modulation using a first spreading code group and one ofsecond spreading codes, the first spreading code group includingspreading codes that have a same repetition period as an informationsymbol period and are used in common by the base stations, the secondspreading codes having a repetition period longer than the informationsymbol period, and being different for each of the base stations, themobile station comprising:

a first table for storing the second spreading code and its phase of atleast one perch channel, which second spreading code and phase areknown;

a second table for storing a second spreading code used by a neighboringbase station;

first search means for searching for a perch channel whose secondspreading code and phase are unknown; and

a second search means for searching for a perch channel whose secondspreading code and phase are known, wherein

the first search means and the second search means carry out theirsearch using the first table and the second table.

Here, the mobile station may further comprise means for transferring,when the first search means captures a perch channel whose secondspreading code and phase are unknown as a result of the search, thesecond spreading code corresponding to the perch channel from the secondtable to the first table.

The second search means may carry out its search using the first table;

the first search means may carry out its search using the second table,and the mobile station may further comprise: means for making decisionof detecting a new perch channel by comparing a search result by thesecond search means with a search result by the first search means.

The first search means may comprise:

means for detecting a peak of a received signal at a timing of a maskedsymbol of the received signal by despreading the received signal usingthe first spreading code group;

means for identifying a group to which the second spreading codebelongs; and

means for identifying the second spreading code.

The second search means may carry out its search using the first table,and the mobile station may further comprise:

third search means for carrying out its search using the second table,the third search means consisting of part of the first search means;

means for deciding detection of a new perch channel by comparing asearch result of the second search means with a search result of thethird search means; and

fourth search means for carrying out its search in response to adecision result, the fourth search means consisting of a remaining partof the first search means.

The direct sequence CDMA mobile communication system may spreadinformation into a signal with a bandwidth broader than a frequencybandwidth of the information using a spreading code sequence with a ratehigher than an information transmission rate.

A direct sequence CDMA mobile communication system may comprise themobile station.

According to the configuration, the power and time required for the cellsearch can be reduced because the first table, which is prepared forstoring captured perch channels, that is, perch channels whose secondspreading code and phase are acquired by the mobile station, enables themobile station to carry out the search for the perch channel only in apredetermined time range (search window) with regard to the phase of theperch channel.

The mobile station usually receives from its visiting base station,information on the scrambling codes used by the neighboring basestations, and carries out the cell search in accordance with theinformation. The present invention is configured such that it transfersthe captured perch channels from the second table to the table ofalready captured perches. This makes it possible to further narrow downcandidates in searching for the perch with the unknown phase, therebysimplifying the search for the perch not only with the known phase butalso with the unknown phase.

Moreover, according to one aspect of the present invention, it appliesthe second search process to the first table; applies to the secondtable a third search process consisting of steps up to a midpoint of thesteps of the first search process; makes a decision of the new perchchannel by comparing the search result of the second search process withthat of the third search process; and obtains the difference between thefirst search process and the third search process on the basis of thedecision result. This enables the mobile station to make full use of thesecond spreading code and phase of each captured perch channel, andhence to make a decision as to whether a new perch channel other thanthe captured perch channel appears or not by carrying out the search ofthe perch channels with the known phases up to the intermediate step 1or 2, without carrying out the entire three steps of the 3-step cellsearch, thereby simplifying the 3-step cell search which is not alwaysessential.

In the third aspect of the present invention, there is provided a cellsearch control method in a CDMA mobile communication system including amobile station which decides a base station the mobile station waits foror communicates with by receiving a perch channel transmitted from thebase station, and which monitors a paging signal to the mobile stationby means of intermittent reception in the idle mode, the cell searchcontrol method comprising the step of:

carrying out, in the mobile station, measurement of receiving quality ofthe perch channel in synchronization with timing of receiving the pagingsignal sent to the mobile station.

Here, the measurement of the receiving quality of the perch channel maybe carried out in the mobile station when a time period counted from alatest measurement of the receiving quality of the perch channel exceedsa predetermined value.

In the fourth aspect of the present invention, there is provided a CDMAmobile communication system including a mobile station communicatingwith a plurality of base stations,

each of the base stations comprising:

perch channel transmitting means for transmitting a perch channel to themobile station; and

paging signal transmitting means for transmitting a paging signal to themobile station, and

the mobile station comprising:

base station decision means for deciding a base station the mobilestation waits for or communicates with through the perch channel byreceiving the perch channel transmitted by the perch channeltransmitting means;

paging signal reception decision means for deciding in an idle mode asto whether the paging signal transmitted to the mobile station by thepaging signal transmitting means is received or not by intermittentreception; and

receiving quality measurement means for measuring the receiving qualityof the perch channel, wherein

the receiving quality measurement means carries out the measurement ofthe receiving quality of the perch channel in synchronization withtiming of receiving the paging signal when the paging signal receptiondecision means decides that the paging signal is received.

Here, the mobile station may further comprise counting means forcounting a time period from a latest measurement of the receivingquality of the perch channel, and the receiving quality measurementmeans may carry out the measurement of the receiving quality of theperch channel when the time period counted by the counting means exceedsa predetermined value.

In the fifth aspect of the present invention, there is provided a mobilestation in a CDMA mobile communication system communicating with aplurality of base stations, the mobile station comprising:

base station decision means for deciding a base station the mobilestation waits for or communicates with through a perch channel byreceiving the perch channel transmitted from the base station;

paging signal reception decision means for deciding in an idle mode asto whether the paging signal transmitted to the mobile station from thebase station is received or not by intermittent reception; and

receiving quality measurement means for measuring the receiving qualityof the perch channel, wherein

the receiving quality measurement means carries out the measurement ofthe receiving quality of the perch channel in synchronization withtiming of receiving the paging signal when the paging signal receptiondecision means decides that the paging signal is received.

Here, the mobile station may further comprise counting means forcounting a time period from a latest measurement of the receivingquality of the perch channel, and the receiving quality measurementmeans may carry out the measurement of the receiving quality of theperch channel when the time period counted by the counting means exceedsa predetermined value.

Thus, in the mobile communication system utilizing the code divisionmultiple access, the base stations employ the same radio frequency.Accordingly, the perch channels and paging channels of the base stationsare all transmitted on the same radio frequency, and are identified bythe spreading codes. This makes it possible to use the radio stage,which receives the radio frequency and extracts the spread modulationsignal, in common to receive the perch channels and paging channels,thereby reducing the uptime of the radio stage by matching the timing ofreceiving these channels.

As described above, because the base stations employ the same radiofrequency, user radio waves of voices or data are also transferred atthe same frequency. This can bring about the degradation in thereceiving quality of the perch channel or paging channel because theuser radio waves become interference in receiving the perch channel orpaging channel.

Furthermore, in the time periods of much interference because of a greatnumber of such user radio waves, the number of paging signalstransmitted from the base stations over the paging channels are alsoincreased. In contrast, in time periods suffering only smallinterference from the user radio waves, the number of paging signalstransmitted from the base stations over the paging channels is alsoreduced. Thus, the receiving quality of the perch channels or pagingchannels depends on the transmission frequency of the paging signals insuch a manner that it is improved with a decrease in the number of thepaging signals, and impaired with an increase in the number thereof.This teaches that to maintain the selection accuracy of the base stationat a high level when the receiving quality of the perch channel is low,it is necessary to increase the frequency of measuring the receivingquality of the perch channel, thereby increasing the quality measurementfrequency.

According to one aspect of the present invention, it is configured suchthat it carries out the quality measurement of the perch channel insynchronization with the received timing of the paging signal. This willautomatically increase the measurement frequency of the receivingquality of the perch channel in the case where the receiving quality islow, thereby improving the selection accuracy of the base station, butdecrease the measurement frequency in the case where the receivingquality of the perch channel is high, thereby saving the powerconsumption. In addition, the present invention is configured such thatit counts the elapsed time from the latest measuring of the receivingquality of the perch channel, and carries out the measurement of thereceiving quality of the perch channel when the elapsed time exceeds apredetermined value. This makes it possible to carry out the measurementof the receiving quality of the perch channel at a minimum frequencyeven when the paging signals are very few such as in midnight.

The above and other objects, effects, features, and advantages of thepresent invention will become more apparent from the followingdescription of embodiments thereof taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating an example of a mobile communicationsystem;

FIG. 2 is a schematic diagram illustrating a method of using spreadingcodes of the mobile communication system;

FIG. 3 is a schematic diagram illustrating relationships of the timingbetween scrambling codes for the signals transmitted from base stationsto a mobile station;

FIG. 4 is a schematic diagram illustrating a structure of a perchchannel;

FIG. 5 is a schematic diagram illustrating a structure of a pagingchannel;

FIG. 6 is a block diagram showing a configuration of a mobile station towhich the present invention is applied;

FIG. 7 is a schematic diagram illustrating an operation in accordancewith the present invention;

FIG. 8 is a schematic diagram illustrating an operational of deciding anew perch;

FIG. 9 is a schematic diagram illustrating an operation of deciding anew perch when using the first and second steps to search for the newperch;

FIG. 10 is a flowchart illustrating an operation of the mobile station602 in FIG. 7;

FIG. 11 is a flowchart illustrating an operation of the mobile stationin FIG. 8;

FIG. 12 is a block diagram showing a configuration of a mobile stationto which the present invention is applied;

FIG. 13 is a flowchart illustrating an operation of a mobile station inaccordance with the present invention;

FIG. 14 is a flowchart illustrating an operation of a mobile station inaccordance with the present invention; and

FIG. 15 is a schematic diagram illustrating an operation state observedon a time axis when the cell search control method in accordance withthe present invention is operating.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The invention will now be described with reference to the accompanyingdrawings.

EMBODIMENT 1

FIG. 6 is a block diagram showing a configuration of a mobile station towhich the present invention is applied.

A mobile station 500 comprises a mobile station transceiver 502, a userinterface 504, a neighboring base station information acquisition andprocessing unit 506, a common controller 508, a cell search controller510, a memory 512, an antenna 514 and a bus 516. In the mobile stationas shown in FIG. 6, only portions associated with the present inventionare illustrated.

The mobile station transceiver 502 demodulates user information andcontrol signals transmitted from the base stations after their radiofrequency modulation, and transmitting user signals and control signalsafter their coding and modulation. The mobile station transceiver 502 isconnected to the antenna 514 and user interface 504.

The common controller 508 carries out the overall control of the mobilestation.

The cell search controller 510 controls the cell search operation withregulating the timing in accordance with neighboring base stationinformation, and stores search results into the memory 512.

The neighboring base station information acquisition and processing unit506 receives and processes scrambling code information about theneighboring base stations sent from the visiting base station, andstores it into the memory 512.

The bus 516 interconnects the common controller 508, cell searchcontroller 510, neighboring base station information acquisition andprocessing unit 506 and memory 512.

FIG. 7 is a schematic diagram illustrating an operation of the presentinvention. The operation of the mobile station 602 in FIG. 7 will now bedescribed with reference to the flowchart of FIG. 10.

As illustrated in the right-hand side of FIG. 7, the mobile station 602captures information about scrambling codes used by its neighboring basestations from the visiting base station, from which the mobile stationreceives the paging information, and stores the information into a codetable 608 of the neighboring base stations in a code table 604 on thememory as illustrated in the left-hand side of FIG. 7 (step S902).

Using the table 608, the mobile station 602 carries out the cell searchfor perch channels with unknown scrambling codes and phases (step S904),creates a table 606 for holding the scrambling codes and phases of thecaptured perch channels (thin solid-line arrows on the right-hand sideof FIG. 7) (step S906); and eliminates the scrambling codes from thecode table 608 of the neighboring base stations (step S908).

As for the perch channels which cannot be captured until then (brokenline arrows on the right-hand side of FIG. 7), both their scramblingcodes and phases are unknown. As described above, the phases correspondto the phase differences between the scrambling codes. In FIG. 7,numerical examples are illustrated of the phase differences between thescrambling codes with regard to the timing the mobile station has as areference, which numerical examples are represented in terms of chips(one chip corresponds to one bit of a spreading code consisting of a bitstream of “0” and “1”).

Moving along the arrow, the mobile station carries out the cell search,and when it can capture a cell, it transfers it to the captured table606 (step S910).

FIG. 8 is a schematic diagram illustrating an operation of making adecision of a new perch. The operation of the mobile station of FIG. 8will be described with reference to a flowchart of FIG. 11.

FIG. 8 illustrates a case which carries out only the first step as thenew perch search, and decides a new perch from its result.

To search for the new perch, the mobile station carries out the firststep of searching for some phase position (step S1002). Apart from this,it performs a window search for reassuring the phases of the capturedperch channels (step S1004). Subsequently, it compares both the results(step S1006); makes a decision that any perch other than the currentlycaptured perches is a new perch channel (step S1008); and identifies thescrambling code by executing the second and third steps for the perchchannel (step S1010).

FIG. 9 illustrates an example of a decision operation of the new perchusing the first and second steps as the new perch search.

It differs from FIG. 8 in that it can make use of the scrambling codegroup information when deciding as to whether the result of the newperch search is a captured perch or a new perch.

In FIG. 9, G1, G7, G15 and G3 each designate a scrambling code group.More specifically, the G1, G7, G15 and G3 denote that the groups, towhich the scrambling codes used for the perches detected at thesephases, are the first, seventh, 15th and third group, respectively.

Comparing their phases with those of the scrambling code groups of thecaptured perches, the mobile station identifies a perch if the phasesdiffer, and then carries out the third step.

In FIG. 9, the perch received at the phase G3 is decided as the newperch because its phase and scrambling code differ from those of thecaptured perches.

In addition, although not illustrated in the drawings, execution of allthe steps from the first to third steps as the new perch search can beimplemented in the same manner as illustrated in FIGS. 8 and 9, exceptthat it can use, for deciding the new perch, both the scrambling codeand phase as the result of the new perch search.

Although the present embodiment is described for the convenience sake asthough the mobile station always carried out the new perch search andcaptured perch confirmation simultaneously, the embodiments inaccordance with the present invention are not limited to this. Forexample, when carrying out both searches for the new and capturedperches regularly, such a configuration can be implemented in whichtheir intervals are set differently so that one of them is carried outat a higher (or lower) frequency than the other. Alternatively, aconfiguration can be implemented in which they can be controlledadaptively to the conditions as needed. As long as the new perchdecision is made by comparing the captured perches with the results ofthe new perch search, a similar effect can be achieved.

As described above, the present invention can provide the control methodof neighboring cell search, and the mobile station, with making use ofthe advantages of the inter-cell asynchronous system, and withminimizing the power consumption and time required for the cell searchby the mobile station without increasing the total cost of the system.

SECOND EMBODIMENT

Next, the second embodiment of the present invention will be describedwith reference to FIGS. 12-15.

FIG. 12 is a block diagram showing a configuration of a mobile stationto which the present invention is applied.

FIG. 12 shows only portions of the mobile station associated with thepresent invention.

A mobile station 300 comprises a radio signal transceiver 302, a trafficchannel transceiver 304, a user interface 306, a perch channel receiver308, a perch channel receiving quality measurement controller 310, apaging signal receiver 312, a common controller 314, a memory 316, anantenna 318 and a common bus 320.

The radio signal transceiver 302 connected to the antenna 318 is adevice for receiving user information and a control signal transmittedfrom a base station after radio frequency modulation, and fortransmitting user information or control information about voices ordata to be transmitted from the mobile station to the base station.Although it is integrally illustrated in FIG. 12, the transmitter andreceiver can be provided separately. The traffic channel transceiver 304connected to both the radio signal transceiver 302 and user interface306 is a device for carrying out codec of the user information such asvoice or data. The perch channel receiver 308 measures the receivingquality of the perch channel, and extracts broadcast information fromthe base station by decoding the perch channel. The perch channelreceiving quality measurement controller 310, issuing a command to theperch channel receiver 308, controls the measurement operation of thereceiving quality of the perch channel. The paging signal receiver 312receives and decodes a paging signal sent from the base station. Thecommon controller 314 carries out the overall control of the mobilestation, and the memory 316 is used for storing various items ofinformation. The common bus 320 interconnects the perch channel receiver308, perch channel receiving quality measurement controller 310, pagingsignal receiver 312, common controller 314 and memory 316.

Next, the operation of the mobile station with such a configuration willbe described with reference to FIGS. 13 and 14.

As described above, the intermittent receiving technique is applied tothe mobile station. Besides, as disclosed in the foregoing paper“Japan's Revised Proposal for Candidate Radio Transmission Technology onIMT-2000: W-CDMA Revised Proposal Version 1.1”, a great number of mobilestations are divided into a plurality of groups, and each paging signalassigned to one of the groups is mapped onto a single physical channelto configure the paging channels. FIG. 5 illustrates a paging signalassigned to one of the groups. In FIG. 5, reference symbols PIs eachdesignate a very short signal informing whether paging is present ornot; and MUIs each designate a portion including paging information (IDnumber of mobile station). The mobile station receives the PI portion,first, and then the MUI portion only when a decision is made that thepaging is present from the receiving result of the PI portion.

FIG. 13 is a flowchart illustrating the operation of the mobile stationin accordance with the present invention.

The mobile station decides as to whether a timing for receiving the PIcomes, first (step S1402), and when the timing comes, it activates theradio signal transceiver 302 to receive the PI portion in the pagingsignal (step S1404). When it decides that the paging is not present fromthe receiving result, it stops the operation of the radio signaltransceiver 302 (step S1414). In contrast, when it decides that thepaging is present, it continues to operate the radio signal transceiver302 (step S1406). At the same time, it commands the paging signalreceiver 312 to receive the paging signal (step S1408), and causes theperch channel receiving quality measurement controller 310 to issue thecommand to the perch channel receiver 308 to measure the receivingquality of the perch channel (step S1410). Subsequently, it decideswhether the individual operations have been completed (step S1412),stops the operation of the radio signal transceiver 302 when completed(step S1414), and waits until the next timing of receiving the PI comes(step S1402).

FIG. 14 is a flowchart illustrating the operation of the mobile stationwith a configuration of carrying out the next quality measurement at thetime when a predetermined time period has elapsed after the previousquality measurement.

The mobile station decides as to whether a timing for receiving the PIcomes, first (step S1502), and when the timing comes, it activates theradio signal transceiver 302 to receive the PI portion in the pagingsignal (step S1504). When it decides that the paging is not present fromthe receiving result, it proceeds to the decision step of the elapsedtime from the latest quality measurement (step S1508). When the elapsedtime has exceeded the predetermined value, it carries out the sameoperation as when the decision is made that the paging is present (stepS1506). When the elapsed time is below the predetermined value, it haltsthe operation of the radio signal transceiver 302 (step S1516), andwaits for the next timing (step S1502). In contrast, when it decidesthat the paging is present at step S1504, it continues to operate theradio signal transceiver 302 (step S1506). At the same time, it commandsthe paging signal receiver 312 to receive the paging signal (stepS1510), and causes the perch channel receiving quality measurementcontroller 310 to issue the command to the perch channel receiver 308 tomeasure the receiving quality of the perch channel (step S1512).Subsequently, it decides as to whether the individual operations havebeen completed (step S1514), stops the operation of the radio signaltransceiver 302 when completed (step S1516), and waits until the nexttiming of receiving the PI comes (step S1502).

FIG. 15 is a schematic diagram illustrating an operation state observedon a time axis when the cell search control method in accordance withthe present invention is operating.

In FIG. 15, the top view illustrates the quality measurement of theperch channel, and the bottom view illustrates the paging reception.Shadowed portions in the top view denote portions of executing thequality measurement of the perch channel. The bottom view illustratesthe paging reception taking an example of a paging channel in which apaging signal consists of the PIs (narrow portions) and MUIs (wideportions). When the PIs indicate the presence of the paging information,the corresponding MUIs are denoted as a shadowed portion, whereas whenthey indicate the absence of the paging information, the MUIs aredenoted as a blank. Thus, the mobile station does not receive the blankMUIs because they have no paging information.

As illustrated in FIG. 15, according to the present invention, thereceiving quality measurement of the perch channel is controlled inaccordance with the presence or absence of the paging information.Specifically, when the paging information is present, the measurement ofthe perch channel receiving quality is executed at the same time as thepaging reception, whereas when the paging information is absent, themeasurement of the perch channel receiving quality is skipped.

As described above, the embodiment according to the present invention isconfigured such that the mobile station controls the timing of measuringthe receiving quality of the perch channel in synchronization with thepaging signal to the mobile station so that the measurement of thereceiving quality of the perch channel is carried out simultaneouslywith the reception of the paging signal. This makes it possible to savethe power consumption with maintaining the high accuracy of selectingthe best base station.

Furthermore, the embodiment is configured such that it counts theelapsed time from the measurement of the receiving quality of the perchchannel, and when the elapsed time exceeds the predetermined value, itcarries out the measurement of the receiving quality of the perchchannel. This makes it possible to maintain the accuracy of selectingthe best base station at a higher accuracy, making is possible tofurther reduce the power consumption.

OTHER EMBODIMENTS

As to the standards of the third generation mobile communication system,IMT-2000 (International Mobile Telecommunications-2000), the 3GPP (ThirdGeneration Partnership Project) is making a plan. Details of paginginformation transmission method is described in the standard “3GTS25.211 V3.3.0”. To increase the versatility of the standard, it ismodified slightly from the “Japan's Revised Proposal for Candidate RadioTransmission Technology on IMT-2000: W-CDMA”. Specifically, it isconfigured such that the information about the presence and absence ofthe paging is transmitted over a PICH (Paging Indicator CHannel), andthe paging information itself is transmitted over an SCCPCH (SecondaryCommon Control Physical CHannel). Although the physical configuration isthus modified, the standard is the same as the present specification inthe procedure for the mobile station to receive the PI portion beforereceiving the paging information, and to receive the paging informationitself only when a decision is made that the paging is present as aresult of receiving the PI, and in the effect of the intermittentreception obtained from the procedure. Accordingly, it will be obviousfor those skilled in the art that the present invention is applicable tothe “3GTS 25.211 V3.3.0”. In addition, it is obvious for those skilledin the art that the present invention is not limited to the radioschemes described above, but can be implemented in any radio schemesutilizing the present invention.

Moreover, although the foregoing embodiments handle the case in whichthe embodiments are implemented independently, the present invention isnot limited to this. For example, any proper combination of theforegoing embodiments can be implemented, which will be obvious to thoseskilled in the art.

What is claimed is:
 1. A control method of searching for a neighboringcell of a mobile station communicating with base stations in a directsequence CDMA mobile communication system which transmits information bycarrying out double modulation using a first spreading code group andone of second spreading codes, the first spreading code group includingspreading codes that have a same repetition period as an informationsymbol period and are used in common by the base stations, the secondspreading codes having a repetition period longer than the informationsymbol period, and being different for each of the base stations, saidcontrol method of searching for a neighboring cell comprising: a step ofstoring into a first table the second spreading code and its phase of atleast one perch channel, which second spreading code and phase areknown; a step of storing a second spreading code used by a neighboringbase station into a second table; a first search step of searching for aperch channel whose second spreading code and phase are unknown; and asecond search step of searching for a perch channel whose secondspreading code and phase are known, wherein said control method ofsearching for a neighboring cell carries out the first search step andthe second search step using the first table and the second table. 2.The control method of searching for a neighboring cell as claimed inclaim 1, further comprising the step of transferring, when capturing aperch channel in the first search step of searching for a perch channelwhose second spreading code and phase are unknown, the second spreadingcode corresponding to the perch channel from the second table to thefirst table.
 3. The control method of searching for a neighboring cellas claimed in claim 1 or 2, further comprising the steps of: carryingout the second search step using the first table; carrying out the firstsearch step using the second table; and detecting a new perch channel bycomparing a search result at the second search step with a search resultat the first search step.
 4. The control method of searching for aneighboring cell as claimed in claim 1, wherein said first search stepcomprises: a step of detecting a peak of a received signal at a timingof a masked symbol of the received signal by despreading the receivedsignal using the first spreading code group; a step of identifying agroup to which the second spreading code belongs; and a step ofidentifying the second spreading code.
 5. The control method ofsearching for a neighboring cell as claimed in claim 4, comprising thesteps of: carrying out the second search step using the first table;carrying out a third search step using the second table, the thirdsearch step consisting of part of sub-steps constituting the firstsearch step; deciding detection of a new perch channel by comparing asearch result of the second search step with a search result of thethird search step; and carrying out a fourth search step in response toa decision result, the fourth search step consisting of sub-steps of thefirst search step, which are not carried out in the third search step.6. The control method of searching for a neighboring cell as claimed inclaim 1, wherein said direct sequence CDMA mobile communication systemspreads information into a signal with a bandwidth broader than afrequency bandwidth of the information using a spreading code sequencewith a rate higher than an information transmission rate.
 7. A mobilestation communicating with base stations in a direct sequence CDMAmobile communication system which transmits information by carrying outdouble modulation using a first spreading code group and one of secondspreading codes, the first spreading code group including spreadingcodes that have a same repetition period as an information symbol periodand are used in common by the base stations, the second spreading codeshaving a repetition period longer than the information symbol period,and being different for each of the base stations, said mobile stationcomprising: a first table for storing the second spreading code and itsphase of at least one perch channel, which second spreading code andphase are known; a second table for storing a second spreading code usedby a neighboring base station; first search means for searching for aperch channel whose second spreading code and phase are unknown; and asecond search means for searching for a perch channel whose secondspreading code and phase are known, wherein said first search means andsaid second search means carry out their search using said first tableand said second table.
 8. The mobile station as claimed in claim 7,further comprising means for transferring, when said first search meanscaptures a perch channel whose second spreading code and phase areunknown as a result of the search, the second spreading codecorresponding to the perch channel from the second table to the firsttable.
 9. The mobile station as claimed in claim 7 or 8, wherein saidsecond search means carries out its search using the first table; saidfirst search means carries out its search using the second table, andwherein said mobile station further comprises: means for making decisionof detecting a new perch channel by comparing a search result by saidsecond search means with a search result by said first search means. 10.The mobile station as claimed in claim 7, wherein said first searchmeans comprises: means for detecting a peak of a received signal at atiming of a masked symbol of the received signal by despreading thereceived signal using the first spreading code group; means foridentifying a group to which the second spreading code belongs; andmeans for identifying the second spreading code.
 11. The mobile stationas claimed in claim 10, wherein said second search means carries out itssearch using said first table, and where in said mobile station furthercomprise: third search means for carrying out its search using saidsecond table, said third search means consisting of part of said firstsearch means; means for deciding detection of a new perch channel bycomparing a search result of said second search means with a searchresult of said third search means; and fourth search means for carryingout its search in response to a decision result, said fourth searchmeans consisting of a remaining part of said first search means.
 12. Themobile station as claimed in claim 7, wherein said direct sequence CDMAmobile communication system spreads information into a signal with abandwidth broader than a frequency bandwidth of the information using aspreading code sequence with a rate higher than an informationtransmission rate.
 13. A direct sequence CDMA mobile communicationsystem comprising the mobile station as claimed in claim 7.